1. Field of the Invention
The present invention relates to electrical circuits used in digital magnetic recording devices, such as digital magnetic tape drives, magnetic disk drives, and the like, and, more particularly, relates to circuits used to convert electrical signals from a magnetic read head to digital data.
2. Description of the Related Art
Digital tape drives are commonly used to store large amounts of computer generated digital data onto a magnetic recording tape. For example, such tape drives are used to backup large amounts of computer data stored on hard disk drives to provide compact long-term storage of the data.
Digital data (1's and 0's) are stored on tape and other magnetic media (such as disk drives) by transitions in a magnetic field generated by a write head proximate to the surface of the magnetic media. The transitions in the magnetic field cause the magnetic poles of particles on the surface of the recording media to become aligned in accordance with the direction of the magnetic field generated by the write head. Digital 1's and 0's are thus represented on the magnetic media by the alignments of the magnetic poles of the particles. A number of different encoding schemes are used to translate the digital 1's and 0's into magnetic field transitions and thus to particle alignments on the magnetic media. Some examples of such encoding schemes are return to zero (RZ) encoding, non-return to zero (NRZ), non-return to zero inverted (NRZI), phase encoding (PE), and the like.
When the data is to be recovered from the magnetic recording media, the media is moved past a read head that is also positioned proximate to the media surface. The read head senses transitions in a magnetic field caused by the alignments of the magnetic particles on the media surface as it passes by the read head. The read head generates a small electrical voltage that is responsive to the magnetic field transitions. The generated head voltage is amplified and is provided to an associated read circuit which converts the head voltage to digital data that corresponds to the original digital data that was encoded onto the magnetic media. The read circuit accomplishes this by sensing the peaks in the head voltage that correspond to the magnetic field transitions. The peaks are sensed by differentiating the amplified head voltage to provide a differential pair of differentiated voltages that have equal magnitudes at times that correspond to the peaks of the amplified head voltage. The times at which the equal magnitudes occur are detected by threshold detection circuitry within the read circuit, and the timing of the occurrences of equal magnitude are used to determine whether the data to be reproduced is a digital 1 or a digital 0. The times when the voltages of the pair of differential voltage have equal magnitudes are commonly referred to as the "zero-crossings" of the pair of voltages, although one skilled in the art will understand that at the "zero-crossings", the pair of voltages have a non-zero magnitude.
In order to successfully reproduce the digital data in accordance with the above-described technique, it is important that the read circuit accurately locate the zero-crossings of the differentiated voltage. Typical tape read circuits use differential amplifier circuits to amplify the signals from the read head to a usable signal level. Other amplifier circuits are used to amplify the differentiated signal prior to the threshold detection circuitry. Because of imperfections in the amplifier circuits, each stage of amplification has the potential of introducing errors in the amplified signals that can cause the threshold detection circuitry to provide an output signal having errors in the time locations of the zero-crossings of the differentiated signal. These errors can be caused, for example, by amplifiers having differential voltage offsets in their differential output voltages when the offset should be zero and/or having a common mode voltage offset on the differential outputs. For high data densities on the magnetic media, small errors in the timing of the zero-crossing signals can prevent the accurate reproduction of the digital data.
Known magnetic recording and playback devices include compensation circuitry to adjust the amplifier stages to reduce the differential and common mode voltage offsets. However, known compensation circuitry generally includes a manual adjustment devices, such as a potentiometer, or the like, that are adjusted in the factory to provide desirable offset voltages. However, such adjustments seldom are adequate over a wide temperature range and are further subject to variation with time. Furthermore, manual adjustment of the compensation circuitry is undesirable in a mass produced unit such a magnetic tape drive. Thus, a need exists for an automatic system for adjusting the differential and common mode voltage offsets that does not need an initial manual adjustment and that is self-compensating as components vary with temperature and time.